Microelectronic devices made of semiconducting substrates such as silicon wafers comprise contact areas on the outer surface which are utilized to provide electrical contacts between the microelectronic device and an IC substrate or a printed circuit board. Such contact areas which can be either in the form of contact pads or bump structures usually consist of copper, copper alloys, aluminium or aluminium alloys. In order to provide such an electrical contact soldering and wire bonding are applied.
In both cases the contact areas must be prepared to provide solderability and bondability by depositing metal or metal alloy layers on top of the contact areas which provide functionalities of a barrier layer and a first and/or second bondable surface.
The barrier layer prevents diffusion between the contact areas and the first and/or second bonding layer during soldering or wire bonding. Furthermore, a barrier layer provides mechanical protection to sensitive parts of the microelectronic device beneath the contact areas by providing a hard “shielding layer”. This function is especially demanded in wire bonding operations wherein a thin gold or copper wire is pressed onto the contact pad during bonding.
U.S. Pat. No. 6,815,789 B2 discloses at least one layer comprising at least one of nickel, palladium and alloys thereof. The nickel (alloy) layer serves as a barrier layer. The thickness of said nickel (alloy) layer should be at least 1 μm to provide sufficient barrier properties.
US 2001/0033020 A1 discloses a barrier layer selected from nickel, cobalt, chromium, molybdenum, titanium, tungsten and alloys thereof having a thickness of 0.5 to 1.5 μm.
U.S. Pat. No. 6,445,069 B1 discloses a barrier layer made of nickel having a thickness in the range of 0.5 to 20 μm.
Due to the ongoing miniaturisation of microelectronic devices the distance between individual contact areas (denoted “pitch”) is reduced. At the same time the risk of bridging, i.e. an undesired electrical contact between contact areas is increasing. Bridging is caused by thick barrier layers deposited onto the contact areas.
Wire bonding is associated with application of a force onto a contact area having a bondable surface because the wire is pressed onto the bondable surface during bonding.
When reducing the thickness of a barrier layer such bridging can be minimised. However, at the same time the mechanical protection of sensitive regions beneath the contact area during wire bonding is not sufficient anymore and device failures caused by wire bonding are observed.